The present invention relates to a traveling bridge filter system and more specifically, to a unique method and apparatus for effecting sealing engagement between a backwash shoe and an interior partition wall of a filtration tank.
It is well known in the water and waste water filtration field to utilize tanks divided into a plurality of filter cells, one adjacent the other, and all containing a multi-layer or single layer arrangement of granular filter media such as sand, gravel, and the like. In downward or gravity filtration systems of this type, water or other liquid containing suspended particles is introduced into the filtration tank from above, and filtered water is drawn off from a chamber (also referred to herein as a filtrate channel) either directly beneath, or adjacent and below the individual filter cells. In other words, in some instances, a common filtrate chamber extends beneath all of the filter bed cells. In other instances, and including the preferred arrangement here, the cell partitions extend below the cells to the bottom wall or floor of the tank, and each individual cell has a outlet port or header which permits the flltrate to flow out of the cell and into a common filtrate channel extending along the filter cells, in the longitudinal direction of the tank.
During downward flow through the individual cells, particulate matter is entrapped within the layer or layers of granular filter media. Eventually, however, the particulate matter will clog the filter media, thereby impairing the filtering capability of the system. Thus, there is a need for periodic cleaning of the individual filter cells, typically by way of a backwash operation where backwash liquid is reverse flowed through the filter cells, one after the other, until the entire tank has been backwashed. It is also known to maintain such units in continuous operation during cleaning of individual cells, through the use of a traveling bridge device which moves from one filter cell to the next, backwashing individual cells while permitting the normal filtration process to continue in the remaining cells.
The overall construction of the filtration tank of this invention is generally similar to that described in commonly owned U.S. Pat. No. 4,859,330, incorporated herein by reference. In that patent, a combined backwash/air scour system is disclosed, but the general arrangement of the tank, filter cells and traveling bridge is similar. Generally, the tank is of rectangular shape, with longitudinal side walls and transverse end walls. An interior partition wall extending between the end walls and parallel to the side walls (closer to one side wall than the other) divides the tank into a filtrate channel running along side the cells in the longitudinal direction of the tank. Each cell, defined by transverse partitions extending between the interior partition wall and the other side wall, has an outlet port below the filter media in the cell and communicating with the filtrate channel through the interior partition wall, also referred to herein as the channel wall.
In typical gravity flow filtration systems of this type, the associated traveling bridge includes a collection hood which is adapted to seal against or at least come into close proximity to the partitions forming each cell, and this collection hood serves to carry away the backwash water flowing upwardly through the cell. The bridge also carries a backwash pump which is submerged within the adjacent filtrate channel, and which supplies backwash water to each cell. A backwash shoe mechanism is connected to the pump discharge pipe and includes an outlet which is adapted to align with individual cell outlets. The backwash shoe is held in continuous engagement with the interior partition or channel wall of the tank so that backwash water can be pumped into the individual cell outlets in a direction counter to normal filtration flow, as the bridge moves successively to each cell.
The conventional method of sealing the backwash shoe to the backwash port or cell outlet is by spring tension applied in various ways, to force the shoe against the channel wall. Systems utilizing this design apply this spring tension continuously, causing the backwash shoe (which moves with the traveling bridge), to slide against the stationary wall during the entire backwash operation.
In order to reduce wear caused by the backwash shoe sliding against the wall, wear strips are attached to the backwash shoe and to the channel wall. The wear strips are hard plastic and have a flat sealing face. To effect a seal between the backwash shoe and the cell port or outlet, it is necessary that the sealing surfaces be perfectly flat and in perfect alignment with one another. In addition, sufficient spring tension must be applied to overcome back thrust resulting from the operating pressure applied to the inside of the backwash shoe. Otherwise, the shoe will not seal properly, thereby allowing a portion of the backwash water to bypass into the filtrate channel, adversely effecting the backwash cleaning of the filter media in the cell.
Due to the nature of the operating service of these types of devices, it is inevitable that sand and grit will enter the system and cause accelerated wear between the sliding backwash shoe and the stationary liner on the channel wall. As the mating faces wear, friction between the two components increases, causing an additional side load on the traveling bridge, and increasing the bypassing of backwash water around the mating surfaces of the wear strips.
In addition, the application of spring tension to the backwash shoe transmits an offset load to one side of the traveling bridge mechanism to which the backwash shoe assembly is attached in a cantilevered manner. The offset side load imparted to the bridge can cause problems with bridge tracking on the bridge guide rails, further complicating alignment of the backwash shoe and adversely affecting the sealing of the backwash shoe to the channel wall. Excessive side load imparted to the traveling bridge can even cause derailment of the bridge.
In order to eliminate the problems outlined above, this invention incorporates a positive means for mechanically sealing the backwash shoe to the channel wall. The mechanical means employed will effect a seal only after the backwash shoe and a particular backwash port or cell outlet are in alignment, thereby eliminating problems caused by continuous side loading on the traveling bridge, and also eliminating the need for wear strips on the shoe and on the channel wall. The face of the backwash shoe engaging the channel wall in accordance with this invention, is equipped with a resilient seal face or gasket (shaped to surround the cell outlet in the channel wall) to provide a compression type seal, and to account for minor imperfections in the channel wall.
In the preferred arrangement, an expandable bellows-type actuator is suspended from the bridge with the backwash pump and backwash shoe. The actuator is mechanically connected to the pump discharge pipe adjacent the backwash shoe. The actuator is also fluidly connected by a conduit to the backwash pump discharge. This entire assembly, including the backwash pump, discharge pipe, backwash shoe and bellows actuator, is carried at the lower end of a pair of parallel (in a transverse plane parallel to the cell partitions) support struts which are pivotally secured to the bridge and which extend vertically downwardly into the filtrate channel. In other words, the assembly is relatively loosely hung from the bridge such that the actuator can move the assembly toward the channel wall so that the backwash shoe gasket can sealingly engage the channel wall about individual cell ports in a manner described in greater detail further herein.
When the traveling bridge stops at a cell to be backwashed, the backwash shoe will be aligned with the cell port or outlet in the channel wall as a result of the sensing system which is part of the traveling bridge mechanism, but which is otherwise unrelated to this invention. At this point, the backwash pump is activated to apply flow and pressure to the backwash system. Since the bellows-type actuator is connected to the backwash pump discharge, it is pressurized and expanded smoothly as the pump comes up to operating pressure. The bellows is located such that one side pushes against the tank side wall, thus causing the other side of the bellows to push the backwash shoe toward the channel wall until the resilient gasket is sealed against the channel wall about the cell outlet.
When backwash of the cell is complete, the backwash pump is deenergized, relieving pressure on the bellows and disengaging the seal from the channel wall. This disengaging action of the shoe from the channel wall is assisted by a pair of springs compressing the bellows, and thus pulling the backwash shoe away from the wall. The bridge then travels to the next cell, and the process is repeated until all of the cells are backwashed.
In its broader aspects, therefore, the present invention relates to a traveling bridge filtration system comprising a tank configured to include a plurality of side-by-side filter cells all extending longitudinally in a first direction, and a common filtrate channel extending longitudinally in a second direction perpendicular to the first direction; each filter cell having an outlet extending through an interior wall of the tank thereby establishing fluid communication with the filtrate channel; a traveling bridge mounted atop the tank and movable from cell to cell in the second direction; a backwash pump having an inlet and a discharge pipe, the discharge pipe connected to a backwash shoe, the pump and shoe pivotally suspended from the bridge and located within the filtrate channel; and fluid means for moving the backwash shoe selectively into sealing engagement with a tank wall surface surrounding one of the filter cell outlets, the means responsive to actuation of the backwash pump.
In another aspect, the invention relates to a method of effecting sealing engagement between a backwash shoe and a filter cell outlet port in a traveling bridge filtration system which includes a tank configured to include a plurality of side-by-side filter cells all extending longitudinally in a first direction, and a common filtrate channel extending longitudinally in a second direction perpendicular to the first direction; each filter cell having an outlet extending through an interior wall of the tank thereby establishing fluid communication with the flltrate channel; a traveling bridge mounted atop the tank and movable from cell to cell in the second direction; a backwash pump having an inlet and a discharge pipe, the discharge pipe connected to a backwash shoe, said pump and shoe suspended from the bridge within the filtrate channel, the backwash pump including a discharge pipe connected between the pump and the backwash shoe; the method comprising (a) moving the bridge into alignment with a filter cell to be backwashed; (b) moving the backwash shoe into sealing engagement with a tank wall surface surrounding an outlet of the filter cell as a direct function of pressure developed in the backwash pump; (c) backwashing the filter cell; (d) relieving the pressure in the pump and disengaging the backwash shoe from the tank wall surface; and (e) moving the bridge to the next adjacent cell.
The above described invention has several advantages:
(1) positive sealing is effected between the backwash shoe and the backwash port or cell outlet, eliminating undesirable bypassing of backwash water;
(2) wear strip liners on the backwash shoe and the channel wall are eliminated through the use of an intermittently operated and mechanically actuated resilient seal;
(3) continuous spring mechanisms and resulting side load on the traveling bridge mechanism are eliminated;
(4) improved backwash efficiency is achieved as a result of the superior sealing design which insures all backwash water is directed into the filter bed;
(5) the positive, frictionless, wear-resistant mechanical sealing system actuated by hydraulic power inherent within the system requires no additional power source;
(6) a simplified design is provided with fewer sealing components; and
(7) the system is easily retrofitted to existing systems utilizing the old seal designs.
Other objects and advantages of the invention will become apparent from the detailed description which follows.